Heterochronic Dimorphism of Loxoconcha Uranouchiensis (Ostracoda) and its Implication for Speciation

Abstract

Polymorphism is being actively investigated by ostracode specialists because the phenomenon is widespread and may be a key to understanding speciation patterns and evolutionary processes. However, the methods for recognizing the phenomenon and the cause or causes of polymorphism are still under debate. This study presents an example of genetically controlled ostracode dimorphism in which the relationship between the two forms can be explained in terms of heterochrony. The living males as well as the females of Loxoconcha uranouchiensis in Aburatsubo Cove, central Japan, are found to be dimorphic in size, shape, and surface ornamentation of the carapace. The dimorphic forms appear concurrently in the same microhabitat, suggesting that the dimorphism is genetically rather than environmentally controlled. Although the large and the small forms are indistinguishable in their early stages, morphological differences gradually appear and become complete after the final molting, which is the seventh molting for both forms. The characters determined by the number of moltings, such as the numbers of reticules, bristles, and pores, are identical between adults of the two forms. In contrast, for characters affected by the growth condition of the epidermal-cell system, such as carapace shape and fine surface ornamentation, the small-form adult is similar to the penultimate instar of the large form. Taking the large form as a standard, the small form is paedomorphic. These two forms are reproductively isolated and should be considered different species.

The History of Sexual Dimorphism in Ostracoda (Arthropoda, Crustacea) Since the Palaeozoic

“…The adult stage is termed 'A', whereas juvenile stages are numbered; e.g., 'A-1' (i.e., one stage before the adult) and 'A-2' (two stages before the adult), as shown in Figure 4 [20]. Sexual dimorphism (morphological differences between males and females) is commonly found on carapaces and appendages [4,9,30], as shown in Figures 5 and 6, and is especially recognisable during the last adult stage (A), and to a lesser degree in the later juvenile stages, such as A-1.…”

“…A number of Japanese Cenozoic and living Podocopida genera, such as Finmarchinella, Loxoconcha, Semicytherura and Vestalenula (e.g., Figures 5 and 6), can be recognised by their distinct carapace shape and size sexual dimorphism [2,9,16,17,33]. In many cases, the male forms of these podocopids have relatively longer and narrower carapaces than the females ( Figures 5 and 6) [34][35][36].…”

“…This kind of sexual dimorphism is more distinct in the adult than the juvenile stages [9]. In some cases, the male carapaces may be inflated posteriorly to accommodate relatively large copulatory organs, or it may have a more pronounced posterior keel, whereas in podocopid and myodocopid taxa with brood care, female carapaces are larger and more inflated than in the male [25] [40].…”

“…This is likely why few publications include clear illustrations of ostracod hingements of male and female forms together with the A-1 juvenile stage, especially for ostracod taxa with a complex, rather than simple, hingement morphology; e.g., adont and lophodont hingement types. We know of only one example of a detailed comparison of the number of teeth per gongylodont hingement in adult male and female forms with the A-1 juvenile of Loxoconcha uranouchiensis [9]. Further examples of the sexual dimorphism and paedomorphosis of the complicated hingement type will likely be found in other species or families of podocopid ostracods if hingements of male, female, and A-1 juveniles are precisely examined using SEM.…”

“…For example, in one ostracod order, the Podocopida, there are usually eight moult stages between egg and adult, with the last moulting being the first sexually mature stage. Carapace and appendage sexual dimorphism can be recognised during the last adult stage ( Figures 5 and 6), and to a lesser degree in late juvenile stages [8][9][10]. In organisms-other from ostracods-with abundant fossil records, there are very few cases of likely sexual dimorphism.…”